(a) Early mixed-resolution printing--Print resolution in inkjet printing in the media advance axis is primarily determined by the spacing of the ink orifices, and in normal circumstances the print resolution in the carriage scan axis is the same as in the media advance axis. Classically for example, in the PaintJet and PaintJet XL printers of Hewlett-Packard Company, the print cartridges had a nozzle spacing of 7 to the millimeter (180 nozzles per inch), thereby creating a printing resolution of 7 dots/mm (180 dots per inch, 180 dpi) in the media advance axis, and the print resolution in the carriage scan axis was also 180 dpi. This symmetry made mapping of textual and graphical files, for printing, a relatively straight-forward task.
U.S. Pat. No. 5,541,625 of Holstun et al., however, showed that symmetry was not a necessary constraint--and that asymmetry by factors as high as two could be advantageous. Holstun commented that his invention was "not limited to any particular resolution but can be applied to any addressable pixel grid but can be applied to any addressable pixel grid which increases the resolution in the carriage scan axis."
He then went on, however, to clarify this with the more-specific observation (referring to pixel-per-inch resolution values), "The invention can be used to double the existing resolution (e. g. 360.times.720; 600.times.1200) or to provide a less dramatic increase in resolution (e. a., 300.times.450) . . . " Holstun thus demonstrated that factors between about 1.5 and 2.0 could provide printouts at higher apparent resolution than the cartridge spacing and with little added cost.
At that time, as Holstun pointed out, higher-resolution inkjet printheads were developing with nozzle spacings of 12 to 14 per millimeter (300 and 360 per inch). The demand for higher-quality printing, however, was still not satisfied, and the need existed for improving the overall print resolution without having to further decrease the nozzle spacing on the printhead.
Ordinarily ink spots are inherently circular, or at least roughly so, and this raises the question how two roughly circular spots can be used to fill but not overfill a square area. The Holstun patent makes clear that the ink spots used in his 1/12.times.1/24 mm subpixels are distinctly smaller in diameter than the 1/12 mm height of those subpixels:
"One characteristic of the invention is the feature of placing adjacent ink droplets on the 300.times.600 grid such that greater droplet overlap occurs in the carriage scan direction than in the media advance axis. In that regard, it was discovered that when the ink droplets were excessively small, horizontal banding/white space resulted, thus decreasing the print quality." PA1 (b) Split-reservoir Pens--The previously mentioned patent to Keefe briefly discusses (column 8, lines 28 through 42, and FIG. 9) the possibility of supplying different sectors of a single pen assembly from corresponding different reservoirs within a pen body--a construction that has been denominated a "biphod" pen. Such reservoirs can be formed simply by provision of a dividing wall within an otherwise single, common reservoir structure. PA1 high image quality, PA1 high quality of text even on plain paper, PA1 high throughput of text on plain paper, and PA1 low cost. PA1 black drop volumes of 16 pL or less, for high-quality images; PA1 black ink density of 120 pL or more per pixel in a grid of 12.times.12 dot/mm (300.times.300 dpi), in a single pass, for dark text; PA1 nozzle pitch of 12 dots/mm (300 dpi), for low cost; and PA1 high-quality text. PA1 plural ink-storage chambers within each pen body; or PA1 plural ink-conveyance tubes extending from within each pen body to receive ink from plural external ink sources, respectively.
Thus Holstun seems to warn that the specific embodiments taught in his patent approach a feasibility limit with their 2:1 aspect ratio.
Those skilled in the art may glean, from the various comments excerpted above, that the Holstun invention works at 1.5:1, and perhaps just barely works at 2:1--and that smaller drop diameters leave strips of uncovered white paper. Holding a higher aspect ratio without significant horizontal overfill within a square pixel, however, leads to just such a condition, i. e. smaller drop diameter.
This sort of construction is advantageous for printing at least two ink colors from a single pen. In Keefe the two or more reservoirs feed nozzles formed in different parts of a single, common pen head that is formed by a tape automated bonding (TAB) assembly.
This construction is also known as a TAB head assembly or THA. Important refinements in THA construction are introduced in the Akhavain patent documents mentioned above.
Such an assembly includes internal ink-supply channels and heaters integrally formed in a unitary silicon die, and a polymeric tape bonded to the die. The tape has laser-drilled holes to form the pen nozzles, and electrically conducting traces for interconnecting the heaters with control circuitry elsewhere in the printer.
Pen bodies subdivided in this way are available in routine production, as they have been designed for use in such printers as the Hewlett Packard Model 2000C. The Keefe geometry relies upon a THA with edgewise feed of the ink to the THA--exploiting that feature to achieve a split ink supply with no need for additional complexity of internal channels through central portions of the die.
(c) Prior "staggered"-pen construction--Before introduction of the extremely economical THA, pen nozzle arrays typically were fabricated by drilling nozzle apertures through thin metal plates. In some of those earlier configurations, although plural pen bodies were mutually aligned the individual colors were sometimes staggered.
(d) Present design challenaes--One interesting characteristic of printer-performance criteria is the divergent requirements for images (i. e. pictorial matter such as photographs, paintings and the like) as distinguished from text. With continuing improvement of printer performance and continuing decrease of prices, it becomes more important to design printers that deliver, in combination:
These considerations will be taken up in turn.
High image Quality requires low drop volumes, to reduce the visibility of individual dots on the printing medium. Past inkjet black pens have had drop volumes in the range of 30 to 150 pL. Future black drop volumes will have to be 16 pL or lower to provide high image quality on plain paper.
High text quality on plain paper has been achieved by increasing resolution and maintaining high edge acuity. Previous approaches have led to 24.times.24 dots/mm (600.times.600 dpi) text. High-quality dark text with conventional inkjet inks requires a drop volume of approximately 120 pL per square pixel of 1/12 mm (1/300 inch) on a side--in other words, per pixel in a grid of 12.times.12 dots/mm (300.times.300 dpi).
Highest text throughput is achieved by using single-pass bidirectional printing. At the same time it is necessary to maintain high quality of text appearance, and in single-pass printing this requires that vertically adjacent lines of inkdrops of common color merge.
To accomplish this, the printer must fire the vertically adjacent drops at essentially the same time. In addition--as noted later in this document--the absorptive properties of the printing medium must facilitate such coalescence; fortunately plain paper generally does so.
The previously mentioned copending '927 application of the present inventor and coworker takes up numerous aspects of the multidrop-coalescence problem, particularly from the perspective of single-pass printing as a goal. The '478 application also noted above also introduces several important considerations of multidrop, single-pass printing--but rather from the perspective of hybrid use of that general printing mode--in combination with multipass printing.
Low cost is difficult to achieve for fast, single-pass high-resolution printing. In systems with symmetrical text resolution, a costly, tall, high-density-nozzle-pitch printhead is required.
One way to reduce costs is to use asymmetric text resolution--it is less expensive to fire more drops at higher resolution in the scan axis and use lower resolution (lower-density nozzle spacing) in the paper axis. One example of this approach is taught in the above-mentioned Holstun patent which teaches printing text at 24.times.12 dots/mm (600.times.300 dpi) to achieve high text quality with a nozzle pitch of 12 dots/mm (300 dpi).
The prior art, however, offers no system capable of implementing such specifications.
(e) Conclusion--Absence of such capability continues to impede achievement of uniformly excellent inkjet printing--at high throughput on all industrially important printing media and in particular even on plain paper. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.